1. Field of the Invention
The present invention relates to an optical recording medium allowing information recording by irradiation with a light beam and optical reproduction of thus recorded information, and a method for detecting tracking signals from said medium. The tracking signal detecting method of the present invention is applicable to an apparatus designed solely for information recording, an apparatus designed solely for information reproduction, and an apparatus designed for both information recording and reproduction.
2. Related Background Art
Electronic file systems have been recently developed or commercialized utilizing compact disks or add-on type optical disks, and optical disk systems utilizing erasable magnetooptical or phase transition material. Also, recently attracting attention is an optical card system in which information recording and reproduction are conducted on a card-shaped optical recording medium. Such an optical card is characterized by its portability, because of its shape, and by a large information capacity per area, since a disk-shaped recording medium is unable to record information in its central portion.
The above-mentioned optical recording media record information in the form of an information track or a train of optically detectable recording pits, by scanning the surface of the media with a light beam focused to a small spot and modulated according to the information to be recorded. Thus, in order to correctly record information without trouble, such as crossing of information tracks, it is necessary to control the irradiating position of the light beam in a direction perpendicular to the scanning direction. Such a control function is called auto-tracking. In the following there will be explained a conventional method of detecting a tracking signal for such auto tracking.
FIGS. 1A and 1B are magnified views of the surface of a conventional optical recording medium, wherein reference numeral 13 indicates a tracking track formed in advance on the surface, for example as a stripe-shaped area of different reflectance. Three beam spots 15, 16 and 17 perform a scanning motion in a direction B, in such a manner that the beam spot 16 runs along the center of the tracking track 13, while each of the beam spots 15, 17 half ride the track 13. A tracking signal is obtained from the difference of the detection signals obtained by detecting the light of the spots 15, 17 and is utilized for controlling the beam spots 15, 16, 17, in order to record information in the tracking track 13 with the beam spot 16 as shown in FIG. 1A or reproduce information recorded in the track 13 as shown in FIG. 1B.
In such a conventional method, however, stable auto tracking is often not possible, since record pits 14, indicated by hatched areas, are present in the running trajectory of the beam spots 15, 17 for detecting the tracking signal, so that a signal caused by the record pits 14 may be mixed with the tracking signal. Particularly, in a case of information recording as shown in FIG. 1A, the record pits 14 are always present at the side of the beam spot 15, so that the light from the beam spots 15 and 17 are not balanced and are unable to provide an exact tracking signal.